Analogues of semisynthetic ribonuclease-S' have been made for active site and conformationally important residues in the (1-20) fragment by solid phase peptide synthesis. Analogues for the alpha-helical position 9 have revealed critical correlations between the experimentally observed propensity for alpha-helix formation and empirical propensity parameters (generated by several groups by examination of proteins of known sequence and conformation). Additionally, analogues at the active site His 12 position have been made containing carbon 13 and fluorine 19 atoms. Such analogues, studied by proton and carbon 13 NMR, have helped refine the view of the participation of His 12 in ribonuclease catalysis and, in the case of the enzymically inactive 4-F-His 12 derivative, have provided a vehicle for future studies of the binding of substrates at the active site. Crystalline (normal sequence) semisynthetic ribonuclease-S' has been prepared and X-ray diffraction analyses indicate the structural identity to native ribonuclease-S'. Study of the interaction of bovine neurophysin II with 13C-enriched synthetic oxytocin allowed inferences to be drawn concerning the mode of interaction of the neurohypophyseal hormone with its carrier protein.